---
title: $derived
---

Derived state is declared with the `$derived` rune:

```svelte
<script>
	let count = $state(0);
	let doubled = $derived(count * 2);
</script>

<button onclick={() => count++}>
	{doubled}
</button>

<p>{count} doubled is {doubled}</p>
```

The expression inside `$derived(...)` should be free of side-effects. Svelte will disallow state changes (e.g. `count++`) inside derived expressions.

As with `$state`, you can mark class fields as `$derived`.

> [!NOTE] Code in Svelte components is only executed once at creation. Without the `$derived` rune, `doubled` would maintain its original value even when `count` changes.

## `$derived.by`

Sometimes you need to create complex derivations that don't fit inside a short expression. In these cases, you can use `$derived.by` which accepts a function as its argument.

```svelte
<script>
	let numbers = $state([1, 2, 3]);
	let total = $derived.by(() => {
		let total = 0;
		for (const n of numbers) {
			total += n;
		}
		return total;
	});
</script>

<button onclick={() => numbers.push(numbers.length + 1)}>
	{numbers.join(' + ')} = {total}
</button>
```

In essence, `$derived(expression)` is equivalent to `$derived.by(() => expression)`.